LED lamps are now widely used and LED driving technology is also well known. There are many approaches for driving LEDs in the prior art, including, for example, PWM constant current driving approach and capacitor step-down LED driving circuit. The PWM constant current driving approach provides a good constant control on an output current. Thus, many constant current control chips are commercially available. However, this kind of driving approach is very expensive and very complex, and results in difficulties in circuit design. The capacitor step-down LED driving circuit is simple and cheap, and thus is widely used as a low-power LED lighting driving circuit.
A schematic diagram of a capacitor step-down LED driving circuit according to the prior art is shown in FIG. 1. According to the principle that a capacitive reactance limits a current, an AC power supply Vac drops after passing an input capacitor Cin, and is then supplied to an rectifier circuit 11. The rectifier circuit 11 outputs a DC voltage, which is filtered by an output capacitor Cout and then supplied to a LED load as an output voltage. A resistor R1 is coupled in parallel with an input capacitor Cin, and provides a path for discharging the input capacitor Cin when the AC power supply is turned off.
In the capacitor step-down LED driving circuit according to the prior art as shown in FIG. 1, an output current, which is also a LED driving current, varies greatly with the input capacitor Cin and the AC power supply Vac. Consequently, the LED has an unstable brightness. Moreover, the LED driving current depends on a value of the input capacitor Cin. The LED driving current decreases when a capacitance value of the input capacitor Cin decreases. Consequently, the LED has a decreased brightness. A design scheme of the capacitor step-down LED driving circuit according to the prior art cannot ensure stability of the LED load because the output voltage decreases significantly when the LED load increases. The output capacitor Cout explodes when the AC power supply is excessively high. Moreover, the capacitor step-down LED driving circuit according to the prior art has many safety problems because it lacks surge current limitation, output short circuit protection and open circuit protection.